FIG. 1 shows a section view of an ear with a typical cochlear implant system. A normal ear transmits sounds through the outer ear 101 to the eardrum 102, which moves the bones of the middle ear 103, which in turn excites the cochlea 104. The cochlea 104 includes an upper channel scala vestibuli 105 and a lower channel scala tympani 106 which are connected by the cochlear duct 107. In response to received sounds transmitted by the middle ear 103, the fluid-filled scala vestibuli 105 and scala tympani 106 transmit fluid waves, functioning as an acoustic transducer to generate electric pulses that are transmitted to the cochlear nerve 108, and ultimately to the brain.
To overcome sensorineural hearing loss, a cochlear implant system produces direct electrical stimulation of the cochlea 104. This requires delivery of electrical power from outside the body through the skin to the implanted portion of the system, for example, by inductive coupling through the skin to transfer both the required electrical power and processed audio information for generating the electrical stimulation signals. In FIG. 1, an external transmitter coil 110 is coupled to an external signal processor and placed adjacent to a subcutaneous receiving coil 111 which is coupled to an implanted receiver processor 109. This arrangement inductively couples an audio information-bearing radio frequency (rf) electrical signal to the receiver processor 109. The receiver processor 109 is able to extract from the rf signal both a power component and the audio information.
In addition to extracting the audio information, the receiver processor 109 may also perform additional signal processing such as error correction, pulse formation, etc., and then produces a stimulation pattern based on the extracted audio information that is sent through connecting leads 112 to an implanted electrode carrier 113. Typically, this electrode carrier 113 includes multiple electrodes on its surface that provide selective electrical stimulation of the cochlea 104.
Various signal processing schemes can be implemented to produce the electrical stimulation signals. Signal processing approaches that are well-known in the field of cochlear implants include continuous interleaved sampling (CIS) digital signal processing, channel specific sampling sequences (CSSS) digital signal processing (as described in U.S. Pat. No. 6,348,070, incorporated herein by reference), spectral peak (SPEAK) digital signal processing, and compressed analog (CA) signal processing.
For example, in the CIS approach, signal processing for the speech processor involves the following steps:                (1) splitting up of the audio frequency range into spectral bands by means of a filter bank,        (2) envelope detection of each filter output signal, and        (3) instantaneous nonlinear compression of the envelope signal (map law).Based on the tonotopic organization of the cochlea, each stimulation electrode in the scala tympani is associated with a band pass filter of the external filter bank and symmetrical biphasic current pulses are applied for stimulation. The amplitudes of the stimulation pulses are directly obtained from the compressed envelope signals. These signals are sampled sequentially, and the stimulation pulses are applied in a non-overlapping sequence. Thus, as a typical CIS-feature, only one stimulation channel is active at one time and the overall stimulation rate is comparatively high. For example, assuming an overall stimulation rate of 18 kpps and a 12-channel filter bank, the stimulation rate per channel is 1.5 kpps. Such a stimulation rate per channel usually is sufficient for adequate temporal representation of the envelope signal. The maximum overall stimulation rate is limited by the minimum phase duration per pulse. The phase duration cannot be chosen arbitrarily short, because the shorter the pulses, the higher the current amplitudes have to be to elicit action potentials in neurons, and current amplitudes are limited for various practical reasons. For an overall stimulation rate of 18 kpps, the phase duration is 27 μs, which is near the lower limit. Each output of the CIS band pass filters can roughly be regarded as a sinusoid at the center frequency of the band pass filter which is modulated by the envelope signal. This is due to the quality factor (Q≈3) of the filters. In case of a voiced speech segment, this envelope is approximately periodic, and the repetition rate is equal to the pitch frequency.        
Multichannel cochlear implants not only offer an opportunity to restore some degree of hearing to the profoundly deaf, but also can enable hearing sensations that mimic, at least to some extent, the attributes of normal acoustic hearing. Speech perception results have far exceeded the expectations of early investigators in the field. Taking into consideration that average monosyllabic word scores will reach nearly 80% in the next few years, current research efforts focus on the preservation of residual hearing and on a more natural sound sensation, especially for speech perception in noise and music perception.
Pitch plays a key role in the perception of speech and music, the recognition of a speaker's voice, and in analyzing complex auditory patterns. The two basic cues for pitch perception are the excitation position along the cochlea (place code—see Békésy GV, Experiments In Hearing, New York: McGraw Hill, 1960, incorporated herein by reference) and the temporal patterns of neural excitation (periodicity code—see Wever E., Theory Of Hearing, New York: Wiley, 1949, incorporated herein by reference). It is believed that both the temporal information and the correct tonotopic representation are necessary for complex pitch perception. In electrical stimulation, the perceived pitch depends on rate, level, waveform, and the place of stimulation, namely the position of the electrode. See Townshend B, Cotter N, Van Compernolle D, et al., Pitch Perception By Cochlear Implant Subjects, J. Acoust. Soc. Am. 1987; 82:106-115, incorporated herein by reference.